Central pattern generators are groups of neurons that are responsible for the production of rhythmic motor outputs in vertebrate as well as invertebrate animals. One of the goals of the proposed research is to gain new insights into the mechanisms by which the frequency and phase relations of the motor patterns produced by central pattern generators are modulated. A second goal of the proposed research is elucidate some of the basic properties of peptidergic synapses, and their roles as modulators of neuronal circuits. The neurotransmitter mechanisms underlying the modulation of a central pattern generator of the pyloric system of the stomatogastric ganglion (STG) of decapod crustaceans will be studied. 1. Immunohistochemical and biochemical experiments will be used to define new molecules that are likely to function as neurotransmitters in input fibers to the STG. 2. Physiological experiments will be used to study the action of the neuropeptides, proctolin and FMRFamide, on individual neurons of the STG. 3. Input neurons that contain peptides will be localized, and then electrophysiological experiments will be performed to study the properties of the synapses made by the peptide-containing neurons. 4. The effect of the peptide-containing input neurons on the motor patterns of the STG will be studied. The results of these experiments will be used to evaluate the model that neural inputs to a neuronal circuit that each release a different neurotransmitter each elicit a characteristic and different output from that circuit. These results will aid in understanding why many different neurotransmitters are used in the nervous system.